OVERVOLTAGE PROTECTION

Abstract: The present invention provides output buffers and input/output buffers which block the charge leakage from the bus to the internal power supply when the bus voltage exceeds in magnitude with internal power supply voltage or when the buffer is powered down. An isolation transistor (140) is connected in series with a pull-up transistor (130) between the internal power supply (VDDI) and the output terminal (YIO) which is connected to the bus. A circuit that controls the pull-up transistor (130) in response to an enable signal (EN) and a data input signal (A), controls also the isolation transistor (140) so that when the driver is enabled and the pull-up transistor (130) is on, the isolation transistor (140) is also on allowing the pull-up transistor (130) to drive the ouptut terminal (YIO). Another transistor (146) is provided between the circuit and the gate of the isolation transistor (140) to isolate the gate from the circuit when the driver is disabled. Thus, when the driver is disabled, the circuit does not control the isolation transistor. Instead, the isolation transistor (140) is controlled by a pass-through transistor (156) connected between the gate of the isolation transistor (140) and the output terminal (YIO). When the driver is enabled, the pass-through (156) transistor is off. When the driver is disabled and the voltage on the output terminal exceeds in magnitude a predetermined value, the pass-through transistor (156) turns on to turn off the isolation transistor (140). The isolation transistor (140) is a PMOS transistor in some embodiments. The drain of the isolation transistor (140) is connected to the backgate to turn off the drain/backgate diode.

Description: OVERVOLTAGE PROTECTION BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to overvoltage protection, and more particularly to preventing charge leakage in output and input/output buffers when the voltage on the buffer output exceeds in magnitude the buffer internal power supply voltage or when the buffer is powered down. Description of Related Art Electronic systems sometimes combine modules powered by different supply voltages. For example, in battery powered laptop, notebook and hand-held computers, some modules are powered by a 3.0V or 3.3V voltage while other modules are powered by a 5.0V voltage. The use of the lower power supply voltage such as 3.3V reduces power consumption and thus allows extending the system operation time before the battery must be recharged or replaced. However, modules such as disk drives are powered by 5.0V because they perform better when powered by the higher voltage. If a 3.3V module and a 5.0V module are connected to a common bus and the 5.0V module drives the bus with a 5.0V signal, a charge leakage path may form between the bus and the 3.3V power supply. For example, suppose that the output driver of an input/output buffer of the 3.3V module includes a PMOS pull-up transistor connected between the 3.3V voltage and the bus. When the bus is driven by the 5.0V module, the 3.3V module drives the PMOS transistor gate with 3.3V to turn the transistor off to tri-state the bus. When the bus voltage rises to 5.0V, the PMOS transistor turns on providing a conductive channel between the bus and the 3.3V power supply. Moreover, because the PMOS transistor backgate is typically held at 3.3V, the drain/backgate diode turns on providing another conductive path between the bus and the 3.3V power supply. The undesirable results include loading the bus and causing "bus contention", degrading the bus signals, and causing possible false signals on the bus. Similar problems occur when a selected module is powered down while other modules are powered up, whether or not different power supply voltages are used in the system. When the module is powered down in order, for example, to save power or to replace the module, a leakage path can form between the bus and the module internal power supply. Thus, it is desirable to provide output drivers which do not provide a leakage path between the bus and the module internal power supply. SUMMARY OF THE INVENTION The present invention provides, in some embodiments, output buffers and input/output buffers which block the charge leakage from the bus to the internal power supply when the bus voltage exceeds in magnitude the internal power supply voltage or when the module is powered down. This functionality is achieved as follows in some embodiments. An isolation transistor is connected in series with a pull-up transistor between the internal power supply and the output terminal which is connected to the bus. A circuit that controls the pull-up transistor in response to an enable signal and a data input signal, controls also the isolation transistor so that when the driver is enabled and the pull-up transistor is on, the isolation transistor is also on allowing the pull-up transistor to drive the output terminal. Another transistor is provided between the circuit and the gate of the isolation transistor to isolate the gate from the circuit when the driver is disabled. Thus, when the driver is disabled, the circuit does not control the isolation transistor. Instead, the isolation transistor is controlled by a pass-through transistor connected between the gate of the isolation transistor and the output terminal. When the driver is enabled, the pass-through transistor is off. When the driver is disabled and the voltage on the output terminal exceeds in magnitude the voltage provided by the internal power supply by at least a predetermined value, the pass-through transistor turns on to turn off the isolation transistor. The isolation transistor is a PMOS transistor in some embodiments. The drain of the isolation transistor is connected to the backgate to turn off the drain/backgate diode. Other features of the invention are described below. The invention is defined by the appended claims. BRIEF DESCRIPTION OF THE DRAWINGS Figs. 1-4 are circuit diagrams of input/output buffers according to the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig. 1 is a circuit diagram of input/output buffer 110 which includes output driver 114 and input buffer 118. When driver 114 is enabled by a high signal on the enable terminal EN, driver 114 drives input/output terminal YIO with an output signal logically equivalent to the signal on data input terminal A. When the signal on terminal EN is low, driver 114 tri-states terminal YIO. Buffer 118 amplifies input signals on terminal YIO to provide the amplified signals on terminal Y. Terminal YIO is connected to a bus (not shown) connected to one or more other modules (not shown) . One or more of these modules can drive the bus with a voltage higher than the buffer 110 internal power supply voltage VDDI. In one example, buffer 110 is powered by 3.3V, and one or more other modules can drive the bus with 5.0V. As explained below, driver 114 includes overvoltage protection circuitry to prevent charge leakage from the bus to the buffer 110 power supply when the bus is at 5.0V. Driver 114 includes NAND gate 126 having one input connected to enable terminal EN and another input connected to data input terminal A. In some embodiments, all the logic gates including gate 126 and all the inverters are implemented in CMOS technology. The logic gates and the inverters are powered by the internal supply voltage VDDI (VDD Internal) . The sizes of the transistors of the logic gates and inverters in some embodiments are indicated next to the logic gate or the inverter. Thus, in some embodiments, each P channel transistor of gate 126 has the channel width of 40μm ("wp=40") , and each N channel transistor also has the channel width of 40μm ("wn=40") . The channel lengths are lμm unless shown otherwise. The output of gate 126 is connected to the gate of PMOS pull-up transistor 130. The transistor channel sizes in some embodiments are indicated next to the transistors. Thus, in some embodiments, transistor 130 has the channel width/length dimension of 410μm/lμm. The source of transistor 130 is connected to power supply terminal 136 which receives power supply voltage VDDI. VDDI is 3.3V in some embodiments. The drain of transistor 130 is connected to the source 140.1 of PMOS transistor 140 whose drain 140.2 is connected to terminal YIO. Thus, transistors 130 and 140 are connected in series between terminals 136 and YIO. The backgate of transistor 140 is connected to drain 140.2. The output of NAND gate 126 is connected to one source/drain terminal of NMOS transistor 146 whose other source/drain terminal is connected to the gate of transistor 140. The output of NAND gate 126 is connected in addition to the input of inverter 150 whose output is connected to the gate of transistor 146. The gate of transistor 140 is connected to source/drain terminal 156.1 of PMOS transistor 156. The other source/drain terminal 156.2 of transistor 156 is connected to terminal YIO. The transistor gate is connected to power supply voltage VDDI. The backgate of transistor 156 is connected to terminal 156.2. Enable terminal EN is connected to the input of inverter 160 whose output is connected to one input of NOR gate 164. The other input of gate 164 is connected to data input terminal A. The output of gate 164 is connected to the gate of pull-down NMOS transistor 168 whose drain is connected to input/output terminal YIO and whose source is connected to ground. Input buffer 118 is formed by inverters 176, 180 connected in series between terminals YIO and Y. The backgates of all the PMOS transistors of buffer 110 except transistors 140 and 156 are connected to power supply voltage VDDI. The backgates of all the NMOS transistors in buffer 110 are connected to ground. In some embodiments, the threshold voltage VTN of the NMOS transistors is about 0.7V, and the threshold voltage VTP of the PMOS transistors is about -0.9V. When the enable terminal EN is high, driver 114 is enabled. Gates 126, 164 each invert the signal on data input terminal A. When terminal A is high, the output of gate 126 is low turning on transistor 130. Transistor 146 is on delivering a low voltage to the gate of transistor 140 and thus turning transistor 140 on. Transistor 156 is off. Pull-down transistor 168 is also off, and input/output terminal YIO is driven with a 3.3V signal by transistors 130, 140. When terminal A is low, the output of gate 126 is high turning off transistor 130. Inverter 150 supplies a 0V signal to the gate of transistor 146 turning the transistor off. Transistor 146 isolates the output of NAND gate 126 from source/drain terminal 156.1 of transistor 156 thereby preventing the charge leakage from the output of NAND gate 126 to terminal YIO through the diode formed by source/drain terminal 156.1 and the backgate of transistor 156. Pull-down transistor 168 pulls terminal YIO to 0V. When enable terminal EN is low, transistors 130, 168 are off, and driver 114 is disabled. If the bus is driven with a 5.0V signal, transistors 140, 156 block the charge leakage from the bus to voltage supply VDDI. More particularly, when the voltage on terminal YIO exceeds the voltage VDDI by at least the PMOS threshold voltage so as to possibly turn on transistor 130, transistor 156 turns on raising the voltage on the gate of transistor 140 to the voltage on terminal YIO. Hence the field effect conduction of transistor 140 is suppressed. The conduction through the diode formed by source/drain terminal 140.2 and the backgate of transistor 140 is also suppressed because the backgate is connected to terminal 140.2 so that the diode is not forward biased. Transistor 146 is off protecting NAND gate 126 from the high voltage on the gate of transistor 140. Because the backgate of transistor 156 is connected to source/drain terminal 156.2 rather than to voltage VDDI, the diode formed by terminal 156.2 and the backgate is off so that no leakage occurs through the diode from terminal YIO to voltage supply VDDI. Fig. 2 shows input/output buffer 210 suitable for circuits in which it is forbidden for the gate-to-source and gate-to-drain voltages to reach the value 5.0V. The 5.0V gate-to-source and gate-to-drain voltages are forbidden in some buffers designed for 3.3V operation because the 5.0V voltages can compromise the thin gate oxide in such buffers. In some embodiments, the maximum gate-to-source and gate-to-drain voltage allowed is about 4.2V. Input/output buffer 210 is similar to buffer 110. To prevent the forbidden voltages, buffer 210 includes NMOS transistor 226 connected between source/drain 146.1 of transistor 146 and the gate of transistor 140. The gate of transistor 226 is connected to power supply voltage VDDI. Transistor 226 prevents the voltage drop between the gate of transistor 146 and source/drain 146.1 from reaching the value of 5.0V when enable terminal EN is low and the voltage on the gate of transistor 140 is 5.0V. Similarly, NMOS transistor 232 connected between terminal YIO and the drain of transistor 168 prevents the gate-to-drain voltage of transistor 168 from reaching 5.0V when terminal EN is low (and hence the gate of transistor 168 is at 0V) and the voltage on terminal YIO is 5.0V. The gates of transistors 232, 226 are at voltage VDDI, and hence the gate-to-source and gate-to-drain voltages of transistors 232, 226 are below 5.0V. NMOS transistor 236 whose gate is connected to voltage VDDI prevents the gate-to-source and gate-to- drain voltages in the transistors of inverter 176 from reaching 5.0V. Transistor 236 is connected between terminal YIO and the input of inverter 176. Because the gate of transistor 236 is connected to voltage VDDI, transistor 236 passes at most the voltage VDDI-VTN from terminal YIO to the input of inverter 176. When the voltage on terminal YIO is 3.3V or higher, bootstrap circuit 242 pulls node 246 at the input of inverter 176 to the voltage VDDI to reduce power consumption in the CMOS inverter 176. Bootstrap circuit 242 includes PMOS transistor 250 whose source is connected to voltage VDDI and whose drain is connected to the drain of NMOS transistor 254. The source of transistor 254 is connected to ground. The gates of transistors 250, 254 are connected to node 246. The drains are connected to the gate of PMOS transistor 258 whose source is connected to power supply voltage VDDI and whose drain is connected to node 246. The threshold voltage of the inverter formed by transistors 250, 254 is below VDDI-VTN. For example, in some embodiments, VDDI = 3.3V, VTN = 0.7V, and the inverter threshold voltage is about 60% of VDDI, that is, about 1.98V. Hence when transistor 236 charges node VTN to VDDI-VTN, the inverter drives the gate of transistor 258 to 0V. Transistor 258 then pulls up node 246 to VDDI. Consequently, transistor 250 turns off reducing power consumption. Transistor 236 turns off isolating node 246 from terminal YIO. When the voltage on node 246 is 0V, transistor 258 is off isolating node 246 from power supply voltage VDDI. Input/output buffer 310 of Fig. 3 is similar to input/output buffer 110 of Fig. 1, but the gate of transistor 146 of buffer 310 is connected to power supply voltage VDDI. Inverter 150 of Fig. 1 is eliminated simplifying the circuit. To prevent charge leakage from the gate of transistor 140 to input/output terminal YIO, PMOS transistor 318 is connected in series with transistor 156. More particularly, source/drain terminal 318.1 of transistor 318 is connected to the gate of transistor 140. Source/drain terminal 318.2 of transistor 318 is connected to source/drain terminal 156.1 of transistor 156. Source/drain terminal 156.2 of transistor 156 is connected to input/output terminal YIO. Source/drain terminal 318.1 is connected to the backgate of transistor 318. Source/drain terminal 156.2 is connected to the backgate of transistor 156. The gates of transistors 318, 156 are connected to supply voltage VDDI. Due to the backgate connection of transistors 156, 318, the diode formed by the backgate of transistor 318 and source/drain terminal 318.2 and the diode formed by the backgate of transistor 156 and source/drain terminal 156.1 are connected back-to-back to suppress the diode conduction. Thus, when enable terminal EN is high, the gate of transistor 140 is isolated from input/output terminal YIO since both the diode conduction and the field effect conduction are through transistors 318, 156 are suppressed. When enable terminal EN is low and the voltage on terminal YIO exceeds the supply voltage VDDI by at least the absolute value |VTP| of the PMOS threshold voltage, transistors 156, 318 turn on providing field effect conduction between the gate of transistor 140 and the input/output terminal YIO. Input/output buffer 410 of Fig. 4 is similar to buffer 310 but buffer 410 includes transistors 232, 236 to prevent the gate-to-source and gate-to-drain voltages from reaching 5.0V. Transistors 232, 236 are connected as in buffer 210 (Fig. 2) . Pull-up circuit 242 of Fig. 4 is similar to that of Fig. 2. Buffer 410 does not include transistor 226 of Fig. 2 because the gate of transistor 146 of buffer 410 is connected to supply voltage VDDI and because, therefore, the gate-to-source and gate-to-drain voltages of transistor 146 are below 5.0V. While the invention has been illustrated with respect to the embodiments described above, other embodiments and variations are within the scope of the invention. In particular, the invention is not limited by the transistor sizes or by particular transistor connections. For example, in some embodiments, transistors 140, 130 are interchanged so that the source of transistor 140 is connected to supply voltage VDDI, the drain is connected to the source of transistor 130, and the drain of transistor 130 is connected to input/output terminal YIO. The invention is suitable for output buffers, that is, input buffer 118 is omitted in some embodiments. The invention is not limited by particular voltage values. Thus in some embodiments, voltage VDDI is below ground. In some embodiments, the ground voltage is replaced by another reference voltage. The invention is not limited by a particular fabrication technology. Thus, in some embodiments, the invention is implemented using NWELL CMOS technology in which circuits are formed on a P-doped substrate and the PMOS transistors are formed in one or more NWELLs. In other embodiments, PWELL technology or twin tub technology is used. In some embodiments, the entire input/output buffer is integrated, while in other embodiments discrete components are used. In some embodiments, the bus is integrated with one or more modules. In other embodiments, the bus is not integrated. Other embodiments and variations are within the scope of the invention as defined by the following claims.

Claims: CLAIMS What is claimed is: 1. An output driver comprising: a power terminal for receiving a voltage VI; an output terminal; a circuit Cl having terminals Til and T12 and having a control terminal CT1 for controlling a conductive path between said terminals Til and T12; a circuit C2 having terminals T21 and T22 and having a control terminal CT2 for controlling a conductive path between said terminals T21 and T22, said terminal T21 being connected to said terminal T12, one of said terminals Til, T22 being connected to said power terminal and the other one of said terminals Til, T22 being connected to said output terminal; a circuit C3 for controlling said terminal CT1 in response to an input signal and an enable signal so that: when said enable signal is deasserted, said circuit Cl does not provide a conductive path between said terminals Til and T12; and when said enable signal is asserted, the conductivity of the conductive path between the terminals Til and T12 through said circuit Cl is controlled by said input signal; a circuit C4 for providing a conductive path between said output terminal and said control terminal CT2 such that: when during operation a voltage on said output terminal does not exceed in magnitude the voltage on said power terminal, said circuit C4 does not provide the conductive path between said output terminal and said terminal CT2; and when during operation the voltage on said output terminal exceeds in magnitude the voltage on said power terminal by at least a predetermined value, said circuit C4 provides the conductive path between said output terminal and said control terminal CT2 thereby causing said circuit C2 not to provide the conductive path between said terminals T21 and T22; and a transistor TR for providing a conductive path therethrough between said circuit Cl and said control terminal CT2 so that: when said enable signal is asserted and said circuit Cl provides the conductive path between said terminals Til and T12, said circuit C2 provides the conductive path between said terminals T21 and T22 so that said circuits Cl and C2 connect in series said power terminal to said output terminal; and when said enable signal is deasserted and the voltage on said output terminal exceeds in magnitude the voltage on said power terminal by at least said predetermined value, said transistor TR is off. 2. The driver of Claim 1 wherein when said enable signal is deasserted, said transistor TR is off whatever the voltage on said output terminal. 3. The driver of Claim l wherein said circuit Cl comprises a transistor connected between said terminals Til and T12. 4. The driver of Claim 1 wherein said circuit C2 comprises a transistor TR1 connected between said terminals T21 and T22. 5. The driver of Claim 4 wherein: said transistor TR1 is a field effect transistor; and said driver further comprises means for connecting said terminal T22 and a backgate of said transistor TR1 to said output terminal. 6. The driver of Claim 1 wherein said circuit C4 comprises a field effect transistor TR2 in which a current carrying terminal and a backgate are connected to said output terminal. 7. The driver of Claim 1 wherein said circuit C4 comprises: a transistor TR2 having current-carrying terminals TT21 and TT22; a transistor TR3 having current-carrying terminals TT31 and TT32; means for connecting said terminal TT22 to said terminal TT31; means for connecting said terminal TT21 and a backgate of said transistor TR2 to said output terminal; and means for connecting said terminal TT32 and a backgate of said transistor TR3 to said control terminal CT2. 8. The circuit of Claim 1 further comprising a transistor TR4 connected in series with said transistor TR, said transistor TR4 being connected between said transistor TR and said terminal CT2, said transistor TR4 having terminals TT41 and TT42 and a gate TT43, said driver comprising means for connecting said gate TT43 to said voltage VI, said transistor TR4 being non- conductive during operation when voltages on said terminals TT41, TT42 exceed in magnitude said voltage VI. 9. The driver of Claim 1 further comprising: a reference terminal for receiving a voltage V2 different from VI; a circuit C5 having terminal T51 connected to said reference terminal and having terminal T52, said circuit C5 having a control terminal CT5 for controlling a conductive path between said terminals T51 and T52; a circuit C6 for controlling said terminal CT5 in response to said input signal and said enable signal; and a transistor TR5 connected between said terminal T52 and said output terminal, said driver comprising means for connecting a gate of said transistor TR5 to said voltage VI, said transistor TR5 being non-conductive during operation when voltages on said terminal T52 and said output terminal exceed in magnitude the voltage on the gate of said transistor TR5. 10. An input/output buffer comprising the driver of Claim 1 wherein: said output terminal is an input/output terminal of said input/output buffer; and said input/output buffer further comprises: means for buffering input signals on said input/output terminal; a transistor TR6 having terminals TT61 and TT62 and having a gate TT63; means for connecting said gate TT63 to said voltage VI; means for connecting said terminal TT61 to said input/output terminal; and means for connecting said terminal TT62 to an input of said buffering means, wherein said transistor TR6 is non-conductive during operation when voltages on said terminals TT61 and TT62 exceed in magnitude the voltage on said gate TT63. 11. The input/output buffer of Claim 10 further comprising a circuit connected to the input of said buffering means for pulling said input of said buffering means to said voltage VI when during operation a voltage on said input/output terminal is in magnitude greater than or equal to said voltage VI. 12. An output driver comprising: a data input terminal; an enable terminal; a power terminal for receiving a power supply voltage; a pull-up transistor connected between said power terminal and said output terminal; a circuit for controlling said pull-up transistor, said circuit having one input connected to said data input terminal and another input connected to said enable terminal, said circuit having an output connected to a gate of said pull- up transistor; an isolation transistor connected between said power terminal and said output terminal in series with said pull-up transistor, said isolation transistor isolating said power terminal from said output terminal when during operation a voltage on said output terminal exceeds in magnitude a voltage on said power terminal by at least a predetermined value; means interconnecting said output terminal and a gate of said isolation transistor for turning off said isolation transistor when during operation a voltage on said output terminal exceeds in magnitude a voltage on said power terminal by at least said predetermined value; and a transistor TRl for controllably passing signals produced by said circuit to said gate of said isolation transistor so as to turn on said isolation transistor when said pull-up transistor is on, said transistor TRl being off when the voltage on the gate of said isolation transistor exceeds in magnitude the voltage on said power terminal by at least said predetermined value. 13. The driver of Claim 12 wherein said transistor TRl is connected between the output of said circuit and said gate of said isolation transistor. 14. The driver of Claim 12 further comprising an inverter having an input connected to the output of said circuit and having an output connected to the gate of said transistor TRl. 15. The driver of Claim 12 further comprising a transistor TR2 connected in series with said transistor TRl between said transistor TRl and said gate of said isolation transistor for keeping gate-to-source and gate-to-drain voltages in said driver lower in magnitude than a maximum voltage on said output terminal. 16. A method of preventing a charge leakage between an output terminal and a power supply of an output driver, said method comprising the steps of: when said output driver is enabled and a pull-up transistor is on, connecting a gate of an isolation transistor which is connected in series with the pull-up transistor, to a gate of the pull-up transistor so as to turn on the 15 isolation transistor and to drive the output terminal through the pull-up and isolation * transistors with a voltage provided by said power supply; and when said output driver is disabled or 20 powered down and a voltage on said output terminal exceeds in magnitude the voltage provided by the power supply by at least a predetermined value, disconnecting the gate of the isolation transistor from the gate of the 25 pull-up transistor and connecting the gate of the isolation transistor to the output terminal thereby turning off the isolation transistor. 17. The method of Claim 16 further comprising the 30 step of connecting a backgate of the isolation transistor to the output terminal.

Inventors: DUNCAN RICHARD LAWRENCE, WERT JOSEPH DOUGLAS, DUNCAN, RICHARD, LAWRENCE, WERT, JOSEPH, DOUGLAS
Applicants: NAT SEMICONDUCTOR CORP, NATIONAL SEMICONDUCTOR CORPORATION
Application year: 1994
Main classification: H03K19
IPC codes: H03K17, H03K19